The present invention relates to a crush type pressure detecting device, a rechargeable battery with pressure detecting device, and a portable electronic device. More particularly, it relates to a pressure detecting device capable of detecting deformation due to a pressure change in a simple structure and preventing further danger, and a rechargeable battery and a portable electronic device using the same.
Hitherto, in electronic devices such as portable telephone and video camera, rechargeable batteries (secondary batteries) such as lithium ion battery and nickel-cadmium battery have been widely used. In these types of rechargeable batteries, when falling in overcharged state or short-circuited state, for example, due to trouble or wrong use of electronic device, the inside of the battery is heated, the liquid electrolyte is decomposed to generate gas, and the internal pressure of the battery rises, which may lead to an explosion. To prevent such risk, the rechargeable batteries are provided with various types of pressure detecting devices (refer to Japanese Laid-open Patent No. 11-111263, etc.), and a typical type of such devices is shown in FIG. 22.
In a pressure detecting device 500, a conductor path 520 of a thin film of ceramics or the like is formed on a crush plate 510 which could be easily broken by deformation stress. The pressure detecting device 500 is disposed in contact with the side wall of a housing filled with liquid electrolyte, and each of the electrodes provided at both ends of the conductor path 520 are electrically connected to a current collector and an output electrode in the housing.
In the pressure detecting device 500, when the battery internal pressure increases, the housing expands outwardly, and deformation stress is applied to the crush plate 510. The crush plate 510 is broken when it becomes unable to withstand the stress. As a result, the conductor path 520 is broken, and the charge circuit or discharge circuit which also serves as a protective circuit is cut off. Thus, explosion of the rechargeable battery can be prevented.
In said pressure detecting device, however, since the crush plate 510 forming the conductor path 520 is disposed in direct contact with the side wall of the housing, to enhance the sensitivity of the device, the crush plate 510 itself must be increased in size. Accordingly, using ceramics or the like for the manufacturer of the crush plate 510 has been the cause of increasing the cost. In addition, since the conductor path 520 is formed on the crush plate 510 made of ceramics, its electrical resistance is high, and it has a serious effect on consumption of the battery. On the other hand, there has been a problem that the crush plate 510, if increased in size, becomes easier to warp and is broken even by a slight deformation stress and yet easily affected by drop impact thereby losing its reliability.
Also, alumina ceramics has been used for the crush plate 510, while conductive metal paste comprising a metal of low specific resistance such as silver or copper has been used for the conductor path 520. The burning temperature of alumina ceramics is greatly different from the burning temperature of silver paste or copper paste. Therefore, it is necessary to repeat the burning process two times, that is, burning at the burning temperature (about 1600xc2x0 C.) of alumina ceramics to form the crush plate 510, and the subsequent burning at the burning temperature (about 1000xc2x0 C.) of conductive metal paste to form the conductor path 520. Also, there has been a problem that, to form both the conductor path 520 and the crush plate 510 at one time, it is necessary to use a highly conductive metal such as tungsten whose burning temperature is high, causing considerable increase of the manufacturing cost.
Specifically, when alumina ceramics is used, the plate must be designed to have a thickness of over 0.3 mm to make it hard to warp in burning, and this has been a hindrance to the manufacture of thinner rechargeable batteries. Also, since alumina ceramics is too high in strength, it is necessary to adjust the strength by providing the crush plate 510 with slit 530 as shown in FIG. 22 to make the crush plate easier to break.
The present invention is devised in the light of the problems of the prior art mentioned above, and it is the primary object of the invention to provide highly reliable crush type pressure detecting devices which can be manufactured at low cost.
A crush type pressure detecting device of the invention comprises a crush plate forming a conductor path thereon, a holder for holding the crush plate, and a pair of electrodes conducting with the conductor path. In the crush type pressure detecting device, preferably, the crush plate is provided with an easily broken portion such as V-groove crossing with, for example, the conductor path. The holder holds the crush plate at both sides, or the holder is a plate, and the crush plate is put on this holder.
When the holder is a plate, preferably, a notch is formed in at least a part of the conductor path forming region of the crush plate of the holder. A cover may be placed on the holder by exposing the crush plate.
In the crush type pressure detecting device, preferably, the conductor path is formed in a U-shape. The crush plate is made of ceramics or conductive material, and preferably serves also as a conductor path. The holder is preferably a printed board.
In other crush type pressure detecting device of the present invention, a conductor path is formed on a crush plate which could be broken by deformation stress, and it is characterized in that the crush plate is made of glass ceramics. Preferably an electrode pad is disposed on the other side opposite the conductor path of the crush plate, and there is provided conduction between the conductor path and the electrode through a conductive portion formed inside a through-hole created in said crush plate. It is desirable that another crush plate be placed on the crush plate with the conductor path formed thereon.
Another crush type pressure detecting device of the present invention comprises a crush plate which could be broken by deformation stress, a conductor path formed on said crush plate, and a pair of holders which hold at the sides the crush plate, and it is characterized in that the crush plate is made of glass ceramics. It is preferred to use, as the holder, a thin plate such as a printed circuit board, a glass ceramic plate and an alumina ceramic plate. Then preferably the electrode pad is disposed on the printed circuit board and there is provided conduction between the electrode pad and the conductor path through the circuit on the printed circuit board. It is desirable that the length of the crush plate be set to less than ⅓ of the overall length of the crush type pressure detecting device.
In these crush type pressure detecting devices, preferably the conductor path is formed by burning conductive metal paste whose burning temperature is nearly the same as the crush plate burning temperature.
A rechargeable battery with pressure detecting device of the present invention comprises a rechargeable battery provided with a positive or negative collector inside a case and with an output electrode electrically connected to said collector and a crush type pressure detecting device, and it is characterized in that the collector and the output electrode are electrically connected to each other through said crush type pressure detecting device. In the rechargeable battery with pressure detecting device, a laminate film case is preferably used.
The portable electronic device of the invention is a portable electronic device having a rechargeable battery, and the rechargeable battery is the rechargeable battery of the invention.